A hybrid electric vehicle is known from WO 2013/067292 having a dual energy storage system comprising a high voltage power system and a low voltage power system that are selectively connectible via a DC to DC converter.
Although the hybrid vehicle disclosed in WO2013/067292 has improved fuel economy compared to a vehicle having a single electrical power system due to the ability of the two electrical power systems to be used to capture energy with no fuel penalty transfer energy from the high voltage system to the low voltage system and use energy so captured to assist the engine the inventors have realised that further fuel savings can be achieved with no significant additional cost by operating such a system in a different manner.
It is an object of the invention to provide a mild hybrid electric vehicle having improved fuel economy by optimising the energy storage and utilisation efficiency of the two electrical power systems.